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HIGHLIGHT www.rsc.org/materials | Journal of Materials Advances in Li–S batteries

Xiulei Ji and Linda F. Nazar*

DOI: 10.1039/b925751a

Rechargeable Li–S batteries have received ever-increasing attention recently due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li ion batteries based on intercalation reactions. Li–S batteries may represent a next-generation system, particularly for large scale applications. The obstacles to realize this high energy density mainly include high internal resistance, self-discharge and rapid capacity fading on cycling. These challenges can be met to a large degree by designing novel sulfur electrodes with ‘‘smart’’ nanostructures. This highlight provides an overview of major developments of positive electrodes based on this concept.

Introduction cost factors. These will undoubtedly move the overall redox couple, described by the + beyond intercalation chemistry into the reaction S8 + 16Li + 16e 4 8Li2S, lies at Following two decades of optimization, realm of ‘‘integration’’ chemistry where an average voltage of 2.15 V with respect Li ion batteries (LIB’s) are approaching discharge/charge of the electrode is to Li+/Li. This potential lies about 1/2 to the energy density limits that intercalation coupled with cleavage/formation of 2/3 of that exhibited by intercalation materials can potentially provide, of covalent bonds together with morpho- positive electrodes. However, this is offset 1 about 300 mA h g . At present, the LIB logical/structural dynamics during redox. by the theoretical specific capacity of 1672 cannot offer a suitably long driving range Examples include the electrochemical mA h g1 afforded by the non-topotactic (i.e., >300 km) for pure electric vehicles reactions that provide the basis for integration process, the highest value for (PEV’s), and it poses limitations for plug- Li–Air1 and Li–S batteries. all known solid cathode materials. Thus, in hybrid electric vehicles (PHEV). Large- Herbet and Ulam first introduced the compared to conventional LIB, Li–S scale, high energy density and inexpensive concept of elemental sulfur as a positive batteries have the opportunity to provide energy storage systems are also required electrode material in 1962.2 Sulfur has a greatly increased energy density at for load leveling for renewable energy many valuable characteristics, such as low a lower cost (see Table 1). Theoretical sources. New systems are being sought for equivalent weight, extremely low cost, and values can approach 2500 W h kg1 or the next-generation batteries to provide nontoxicity. Considerable efforts have 2800 W h l1 on a weight or volume basis, Downloaded by on 07 March 2012 much higher energy density, and reduce been devoted to alkali metal–sulfur energy respectively,4 assuming complete reaction

Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A 3 storage systems such as Na–S batteries to Li2S. University of Waterloo, Department of which operate at 300–350 C, and room The earliest configuration of a Li–S Chemistry, WaterlooOntario, Canada N2L temperature Li–S batteries. In a Li–S cell, battery was presented in the late 1960s.5,6 3G1. E-mail: [email protected]

Xiulei (David) Ji received his Linda Nazar is Professor of B.Sc. in Chemistry from Jilin Chemistry and Electrical Engi- University in 2003. He obtained neering at the University of his Ph.D. in Materials Chem- Waterloo, Waterloo, Ontario, istry from the University of Canada and holds a Senior Can- Waterloo in 2009, under the ada Research Chair in Solid State supervision of Professor Linda Materials. She received her F. Nazar. He has been awarded Ph.D. in Chemistry from the a Natural Sciences and Engi- , and then neering Canada Postdoctoral joined Exxon Corporate Fellowship that he will take up at Research (USA) as a post- the University of California, doctoral fellow. She was awarded Santa Barbara in 2010. His the Society Xiulei Ji research interests focus on Linda Nazar Battery Research award in 2009, nanostructured materials and and was the 2010 Moore Distinguished Scholar at the California their applications in energy Institute of Technology. Her research is focused on materials for storage and conversion. energy storage and conversion, with research spanning Li-ion and Na-ion batteries, Li-sulfur and Li-air batteries, and fuel cell catalysts.

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Table 1 Comparison of a typical C/Li[Co1/3Ni1/3Mn1/3]O2 battery and the Li–S battery

Average Theoretical Practical ‘‘Bare-bones’’ energy ‘‘Practical’’ specific discharge capacity of capacity of density of a full energy density of a System potential/V cathode/mA h g1 cathode/mA h g1 cella/W h kg1 full cell/W h kg1

e b C–LiMO2 3.7 275 160 410 135–180 Li–S 2.15 1672 500–1100 950–1700 350c to 700d a Based on active mass (anode + cathode) only—excluding , separator and components. b Reported values for full cells from various sources. c d e Data reported by Sion Power. Based on company projections, and estimated from data in ref. 29. Li[Co1/3Ni1/3Mn1/3]O2.

2 believed to be S8 .The reaction sequence the coulombic efficiency in the charge 13 on reduction and oxidation and the cor- process. It is even observed in Li/TiS2 responding electrochemical profiles are cells.14 On the other hand, the shuttle summarized in Fig. 1. Note that the sulfur provides intrinsic overcharge tolerance active mass (d ¼ 2.03 g cm3) expands for Li–S batteries.15 Mathematical during discharge owing to the lower modeling has yielded a good quantitative 3 16 density of Li2S (1.67 g cm ), and understanding of the phenomenon. It contracts again on charge: an important encompasses theoretical models of the factor to consider when designing charge process, charge and discharge a composite electrode. capacity, thermal effects, self-discharge, Despite the considerable advantages of and a comparison of simulated and Fig. 1 Discharge–charge profiles of a Li–S cell, illustrating regions (I) conversion of solid the Li–S cell, it presents many challenges. experimental data. The study provides 10 sulfur to soluble polysulfides; (II) conversion of The first is the insulating nature of evidence that self-discharge, charge–

polysulfides to solid Li2S2; (III) conversion of sulfur and its final discharge products discharge efficiency, and overcharge solid Li2S2 to solid Li2S. which necessitates contact with substan- protection are all facets of the same tial fractions of conductors. The second is phenomenon. Finally, on the completion the solubility of the long chain polysulfide of each discharge process, the soluble 2 The positive electrode comprised ions (Sn ) formed on reduction of S8 or polysulfides are reduced and deposit as elemental sulfur, electronic conductors upon oxidation of the end-member Li2S2/Li2S precipitates on the cathode. (carbon or metal powder) and binders, sulfides, which is especially trouble- Insoluble agglomerates are thus formed separated from the metallic some.11 These molecular species typically on the surface over prolonged cycling negative electrode by an organic electro- diffuse in solution through the separator regardless of the initial cathode 17,18 Downloaded by University of Waterloo on 07 March 2012 lyte. This configuration has been the to the lithium negative electrode where morphology. The agglomerates

platform for subsequent major research they are reduced to insoluble Li2Sor become electrochemically inaccessible, Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A 8,12 activities as well. In an organic electrolyte, Li2S2. Once the Li anode is fully causing active mass loss at the cathode 2 discharge of the sulfur cell proceeds coated, the following Sn react with these side and the build-up of impedance layers through a three stage process.7,8 In the fully reduced sulfides to form lower order that results in capacity fading. 2 high oxidation state regime correspond- polysulfides (Snx ) which become ing to S0 4 S0.5, sulfur is reduced concentrated at the anode side, diffuse Carbon–sulfur electrodes: through a stepwise sequence of soluble back to the positive electrode and are then overcoming the challenges 2 2 polysulfide ions to form S4 . The reac- re-oxidized into Sn . The above parasitic tion kinetics are fast owing to the molec- process takes place repeatedly, creating Carbonaceous materials are extensively ular nature of the species involved. The an internal ‘‘shuttle’’ phenomenon. It used as electronic conductors in the second stage corresponding to the reac- decreases the active mass utilization in the battery industry, and play a particularly 0.5 1 tion of S 4 S to form insoluble Li2S2 discharge process and markedly reduces crucial role in the sulfur electrode. High is hindered by the energy required for carbon content improves conductivity nucleation of the solid state phase. The but at the expense of reduced energy

last stage for interconversion of Li2S2 to density. In the earliest cell configura- Li2S is the most difficult. This conversion tions, bulk carbon and sulfur powder is impeded due to the sluggishness of solid were simply mixed together to form state diffusion in the bulk. The charge macrocomposite electrodes. These cells process is much simpler. In cyclic vol- suffered low capacity, and poor cycling tammograms of sulfur or polysulfide life. Peled et al. first described the electrodes, typically there is only one concept of loading sulfur into the porous anodic peak.9 It is suggested that all the structure of carbon materials to establish polysulfides transform into the interme- more efficient electronic contact and diate with the most facile oxidation Fig. 2 SEM image of MWCNTs added sulfur improve volumetric energy density.19 kinetics (via charge transfer), which is cathode. Reproduced from ref. 24. More recently, applications of activated

9822 | J. Mater. Chem., 2010, 20, 9821–9826 This journal is ª The Royal Society of Chemistry 2010 View Online

material,26 but with a highly homogenous sulfur dispersion as shown in Fig. 3a and b. The diameter of the tubes systemati- cally increases upon sulfur loading, indi- cating the presence of an even coating on their external surface. Reversible capac- ities of up to 700 mA h g1 are reported. However, MWCNT sulfur networks have obvious limitations. First, the surface area and pore volume of CNTs are typi- Fig. 3 SEM images of (a) MWCNTs and (b) MWCNT/sulfur nanocomposites. Reproduced from cally less than 350 m2 g1 and 0.5 cm3 g1, ref. 26. respectively,27 which limit their capacity to accommodate the sulfur active mass. carbons (AC’s) for the sulfur cathode Moreover, the tubes are normally several have been investigated.20 AC’s possess microns long, which may induce discon- high surface area and pore volume, and tinuous sulfur loading. They are further- are also very cost-effective. However, more unfavourable for Li ion transport their pore size distribution is very wide, since ion mobility can only take place ranging from micropores (<2 nm) to along the long CNT axis, and not macropores (>50 nm). The electronic perpendicular to it. Finally, the diameter contact of sulfur encapsulated in the of CNTs, typically several tens of nano- macropores is quite limited, which metres, is larger than optimum. Qiu et al. results in considerable polarization. also studied sulfur/MWCNT composites, When the sulfur mass is large, incom- focussing on MWCNT-core/sulfur-shell plete discharge occurs with S remaining structures.28 Compared to the simple within the core of the particles, particu- mixture of MWCNT’s with sulfur, this Fig. 4 Illustration of the S/C composite larly at high power output.21 core–shell composite (Fig. 4) shows cathode material by using a bimodal porous Substantial advances have been made a higher internal resistance before cycling carbon as the support. MPC represents in the fabrication of nanostructured and a lower initial discharge capacity. mesoporous carbon. Reproduced from ref. 31. carbonaceous materials in the last two However, the core–shell composite decades,22,23 which have been applied to exhibits good cycling stability (670 mA h improving the performance of Li–S g1 after 60 cycles), possible due to a well batteries. Ahn et al. added multi-walled preserved cathode morphology.

Downloaded by University of Waterloo on 07 March 2012 carbon nanotubes (MWCNT’s) to the Most recently, mesoporous carbons sulfur electrode, as depicted in Fig. 2.24 have found applications in Li–S batteries. Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A These act as conducting wires to form To date, the best electrochemical proper- a 3D wire network that encapsulates the ties that have been reported for carbon sulfur particles. The low initial discharge ‘‘contained’’ sulfur systems are exhibited capacity of 480 mA h g1 probably results by ordered interwoven carbon–sulfur from the electrochemically inaccessible composites that comprise high pore- interior of the bulk sulfur particles. volume carbons with 3D-accessible Similar results have been obtained by channel nanostructures.29 Sulfur is readily employing solid carbon nano-fibers as incorporated from the melt by capillary conducting wires.25 Zheng et al. also forces. By this impregnation method, the reported an MWCNT/S nanocomposite percentage of active mass can be precisely controlled. The residual pore volume in the nanocomposite is designed to retain pathways for electrolyte/Li+ ingress and to accommodate the active mass Fig. 5 Schematic of the sulfur confined in the volume expansion during cycling. The interconnected pore structure of mesoporous semi-amorphous and microporous carbon, CMK-3, formed from carbon tubes morphology of the wall structure of that are propped apart by carbon nano-fibers CMK-3 carbon is also conductive to to form channels. The view is down the Li-ion transport. The conductive carbon channels and tubes in cross-section. Lower panels represent subsequent discharging– framework constrains the sulfur within its charging of sulfur with Li, illustrating the channels and generates essential electrical strategy of pore filling to tune for vol- contact as shown schematically in Fig. 5. ume-expansion/contraction. Reproduced from Fig. 6 Schematic showing PEG200 coated Kinetic inhibition to diffusion within the ref. 29. CMK-3/S composites. framework, and the sorption properties

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Additives in sulfur electrodes: enhancement effects

Composites containing sulfur embedded within a conducting polymer were re- ported by Wang et al.33 It is claimed that 2 a full conversion from S8 to S occurs due to the mixture at the molecular level, but the low average potential of 1.6 V suggests a strong overpotential possibly due to limited conductivity. A similar strategy has been explored to form sulfur- conductive polymer composites where organo-sulfur bonds are formed at high temperatures. These composites exhibit stabilized cycling life and rather low capacity.34 To date, the sulfur–polymer cathode strategy has not been highly Fig. 7 Changes in surface morphology of CMK-3/S versus PEG-modified CMK-3/S on cycling. successful, but new approaches are SEM images of CMK-3/S before (a) and after (b) the 15th charge. SEM images of PEG-modified anticipated. CMK-3/S before (c) and after (d) the 15th charge. Reproduced from ref. 29. To immobilize polysulfides, chemical adsorption methods have been investi- of the carbon both aid in trapping charged at all, indicating completely gated. Carbons functionalized by hydro- the intermediate polysulfides. Thus im- blocked transport. Liang et al. reported philic groups act as both absorbents and

mobilized, their full reduction to Li2S2/ a carbon–sulfur nanocomposite based on conductors. These carbons typically can Li2S (or oxidation to S8 on charge) is a hierarchically structured micro–meso- uptake at least 40% Li2S8 at a concentra- achieved within the carbon framework. porous carbon formed by a post-activa- tion of 0.03 M with a weight ratio of 6.2 31 35 Polymer modification of the carbon tion treatment. Sulfur was loaded solely between the adsorbent and Li2S8. Ahn surface (Fig. 6) facilitates a more into the mesopore-wall micropores by et al. reported Mg0.6Ni0.4O nanoparticles complete reaction by providing a chem- a wet-impregnation method, as schemat- (50 nm) for the same function,36 ical gradient that retards diffusion of the ically depicted in Fig. 4. The empty mes- showing that the diffusion of active mass polysulfides into the electrolyte. Revers- opores provide accommodation for the into the electrolyte decreases by 45%. The ible capacities up to 1320 mA h g1 are soluble polysulfide ions formed during results suggest that there may be a critical

Downloaded by University of Waterloo on 07 March 2012 attained with no shuttle phenomenon discharge/charge. Utilization of sulfur is electrolyte polysulfide concentration (99.9% columbic efficiency) on the first strongly affected by the active mass frac- necessary to activate the shuttle Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A cycle, indicating the shuttle mechanism is tion. The composite containing 51.5 wt% phenomenon. More recently, this group

fully suppressed. This polymer modified sulfur exhibits a first discharge capacity of demonstrated that Al2O3 nanoparticles composite clearly exhibits better 818 mA h g1 (obtained at a cut-off have a similar effect.37 With 10 wt% of + morphological control than the unmodi- potential of 1.2 V vs. Li /Li ). Lowering Al2O3 in the sulfur electrode, a stabilized fied composite during cycling, as shown in the loading to 11.7 wt% gives a first discharge capacity of 660 mA h g1 can be Fig. 7. Capacity fading is reduced discharge capacity of 1584 mA h g1 (95% achieved. Visco and Chu have proposed owing to greatly reduced polysulfide sulfur utilization) at a very high current coating the cathode with a mixed ionic concentration in the electrolyte, and the rate of 2500 mA g1. However, rapid electronic conductor (MIEC).38 They materials sustain reversible capacities of capacity fading is observed. reported that the MIEC layer allows 1100 mA h g1 over 20 cycles (Fig. 7). Finally, a very different approach to rapid removal of discharge product Other porous systems have also been C/S composite design involves sputtering precipitates on the cathode. It was also recently reported, including those with a carbon coating (about 18 nm thick) claimed that the overall performance of a pore size less than 3 nm (i.e., strictly directly onto a thin sulfur cathode.32 Li–S cells can be greatly improved by ‘‘super-microporous’’).30 The carbon High initial capacities of 1178 mA h g1 introducing additives containing N–O surface area is dramatically reduced from were achieved, with the capacity fading bonds into cathode and/or electrolyte.39 1473 m2 g1 to 24 m2 g1 in the sulfur- to 500 mA h g1 after 50 cycles. Aurbach et al. recently revealed that this loaded composite at 57 wt% S, which Emphasis was placed on the possible effect primarily lies in the generation of

suggests that the pores are nearly adsorption effect and conductivity a protective film comprising LixNOy and/ completely filled. The electrode exhibits enhancement of the carbon. However, or LixSOy, formed on the Li anode by the an initial discharge capacity of 740 mA h maintenance of the integrity of the reactions of the additive directly with the g1 (obtained at a cut-off potential of coating on cycling was not addressed, anode surface or with other species in 1.5 V vs. Li+/Li), with highly unusual neither whether it functions as a physical the electrolyte i.e., 1,3-dioxolane (DOXL)

cycling behavior. However, the electrode barrier to prevent diffusion of poly- or the electrolyte salt i.e., LiN(SO2CF3)2 containing 75 wt% sulfur cannot be sulfide ions into the electrolyte. (LiTFSI).40

9824 | J. Mater. Chem., 2010, 20, 9821–9826 This journal is ª The Royal Society of Chemistry 2010 View Online

Attention has also been devoted to the cathode provides the lithium source, it Pure ionic liquid have been binder innovation. For example, the opens up the use of nonlithium–metal utilized in Li–S cells as well.62 Cyclic vol- addition of polyvinylpyrrolidone (PVP) anodes such as carbon. This strategy can tammograms exhibit different reduction and polyethyleneimine (PEI) helps main- help decrease the active mass loss on peaks from those in organic electrolytes,

tain cathode morphology by inhibiting the anode side as well. However, Li2S/M which are proposed to be due to the low the agglomeration of lithium sulfide.41 (M: Fe, Co and Cu) electrodes suffer the solubility of polysulfide ions in ionic Complete redox behaviour is realized by problems of all conversion electrodes; liquids. These cells still suffer rapid using gelatin as a binder for nanoscale namely lower gravimetric capacity, and capacity fading from 1300 mA h g1 sulfur/active carbon sphere composites.42 high polarization. He et al. studied during the first cycle to 700 mA h g1 in The electrode shows an initial capacity of a rechargeable cell system with an elec- the 20th cycle. 1 1132 mA h g and retains a reversible trochemically prelithiated sulfur Sulfide glasses such as Li2S–SiS2, and 1 43 capacity of 408 mA h g after 50 cycles. composite as a cathode and graphite as an Li2S–P2S5 are known to be excellent room Further application of a freeze-drying anode.54 A slightly lower working poten- temperature lithium ion conductors.63 method to generate internal macropores tial (by 0.2 V) is observed, compared to These provide much future opportunity increased the capacity to 1235 mA h g1 cells with lithium metal as an anode. This for the design of all-solid-state cells with on the first cycle, and the reversible avenue has promise for addressing the advanced cathode–electrolyte interfaces capacity to 626 mA h g1 due to improved safety concerns of metallic Li negative and improved safety. By using 44 transport. electrodes, although prelithiation has 80Li2S$20P2S5 (mol%) glass as an elec- inevitable drawbacks. Following on these trolyte, Hayashi et al. prepared a solid cell Other configurations of sulfur studies, prelithiated carbon/sulfur cath- where the cathode (sulfur and copper) cells odes have been combined with a gel elec- exhibits a stabilized discharge capacity of trolyte and a Sn–C composite anode, with 650 mA h g1 for 20 cycles.53 Thio-LISI- Another configuration of Li–S batteries is quite promising results.55 Si nanowire CON electrolytes have recently been an ‘‘all liquid’’ system45 that relies on high anodes have also been married with employed in such systems, and exhibit solubility of polysulfides in the electro- lithiated CMK-3/S cathodes.56 Over- good properties.64 lyte. Electrolyte additives are used to charge and higher than expected capacity maintain solubility and increase the frac- fading were observed, which could be due Conclusions tion of active species in solution.46 The to a variety of factors including problems ionic conductivity of these ‘‘catholyte’’ with cathode architecture/composition, Revisitation of Li–S electrochemistry vis cells is very high; however, volumetric or cell imbalance. More work in these a vis rechargeable lithium batteries capacity and stable long-term electro- areas is necessary. demonstrates that new approaches can be chemical performance are compromised. The use of polymer-based electrolytes brought to bear on ‘‘old chemistry’’ to A major issue is the problematic reduc- removes the need for volatile organic excellent effect. However, challenges still

Downloaded by University of Waterloo on 07 March 2012 tion of polysulfides on the Li anode, solvents, leading to significantly remain. High initial capacities above which necessitates the use of an interfacial improved safety. A solid state Li–S cell 1000 mA h g1 sulfur can be achieved by Published on 10 September 2010 http://pubs.rsc.org | doi:10.1039/B925751A ion-conductive membrane as an inhibitor. with a gel-like polymer electrolyte was facilitating electronic and ionic conduc- Alternatively, a solid electrolyte interface first described in 1997.15 Such cells exhibit tivity within the cathode; but mainte- (SEI) can be formed by reacting lithium in good active mass utilization only at nance of the initial cathode morphology is situ with electrolyte or an oxidative elevated temperature (90 C). Recently, difficult. The resulting agglomeration of species in a controlled reaction as Cairns and co-workers have investigated the sulfur active mass occurs not only in described above, and elsewhere.40,47 A rechargeable Li/S cells containing cells with organic liquid electrolytes which polymer protection layer can be formed a variety of polymer-containing electro- can dissolve polysulfides but also in ‘‘all by an ultra-violet curing method.48 The lytes, including polyethylene oxide solid’’ polymer cells.57 The onset of layers formed are microporous, which (PEO),58 polyethylene glycol-dimethoxy- cathode agglomeration substantially allow electrolyte to permeate to the bare ethane (PEGDME),59 and mixtures of lowers the conductivity and the accessi- metal. Fabrication methods for amor- PEGDME or tetraethylene glycol-dime- bility of the active mass, thus leading to phous glass or metal alloy layers have thoxyethane (TEGDME) with ionic capacity fading. Nanoscale or nano- been developed as well.49 liquids.60 Pure polymer electrolytes typi- structured carbon or oxides can be used as

Li2S : Fe mixtures were first investi- cally show poor performance at low inert additives in the sulfur electrode. gated as positive electrode materials by temperatures. The mixture of ionic liquid These additives should at least include 50 Obrovac and Dahn. FeSx phases are and polymer exhibits much better prop- a conducting agent, preferably a chemical formed from the mixture during the erties at low temperatures. Ahn et al. also or physical adsorption agent, and ideally charge process which can be thought of reported improved low temperature an agent which can control the cathode a ‘‘reverse conversion’’ reaction of 4Li + performance of Li/TEGDME/S cells by morphology. Additives, particularly

FeS2 / Fe + 2Li2S. Similarly, mixtures adding DOXL and methylacetate (MA) porous carbons, which can execute 51 61 of Li2S and Co, or Li2S and Cu have into TEGDME to lower the viscosity. a multi-functional task, are highly desir- been reported as active electrodes.52,53 Cu They reported a high initial discharge able. By loading sulfur into nanoporous (or Co) is used as a conducting agent and/ capacity of 1342 mA h g1 with this carbons in a controlled manner, the

or a Li2S decomposition catalyst. Since mixture electrolyte, but poor cycling life. composite electrodes can exhibit high

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